1. Field of the Invention
The present invention relates to the field of radiographic imaging, and in particular, to methods and apparatus for determining the optical density of radiographic imaging film and further enacting corrective adjustments to the optical density of radiographic imaging film used, for example, in mammography.
2. Description of Prior Art
Radiographic imaging of body parts is well known and extremely useful as a diagnostic tool in the medical arts. Radiographic imaging involves positioning a part of a patient to be imaged denoted as the xe2x80x9cstructure of interestxe2x80x9d under an x-ray emitter, exposing the structure of interest to an x-ray beam, and recording the interaction between the x-ray beam and the structure of interest on an image receptor. In most cases, the receptor is radiographic film, and the image is amplified by an intensifying screen. After exposing the structure of interest, the film is removed from the cassette and then developed.
The diagnostic value of radiographic imaging as described is dependent upon the interplay of several factors. One of the most important of these factors is the optical density of the radiographic film. In order to ensure optimal film exposure, typical imaging devices implement an automatic exposure control system (AEC). A typical AEC utilizes a radiation detector for detecting the amount of radiation. The radiation detector is generally disposed adjacent to the radiographic film such that the radiation impinges upon the detector prior to irradiating the radiographic film. In general, the typical energy per photon of the x-ray is sufficient to render the detector transparent or invisible on the radiographic film. Thereafter, the AEC utilizes algorithms and lookup tables to account for the varying properties of the intensifying screen, the radiographic film, and the processor. The AEC then makes the necessary adjustments to the intensity and duration of the radiation. Thus, the AEC is essentially a closed-loop feedback mechanism for optimizing the quality of radiographic images.
Although an AEC is generally sufficient for a standard imaging device, mammography machines present additional problems. First and foremost among these problems is that the typical energy per photon of the x-radiation emitted by a mammography machine is far less than that of other types of imaging devices. Mammography machines emit so-called xe2x80x9csoftxe2x80x9d x-rays that are optimal for detecting subtle differences in the soft tissue found in the human breast. However, soft x-rays present a limitation in that they cannot be transmitted through the radiation detector without imaging parts of the detector on the radiographic film. Thus, the detector must be disposed such that the radiographic film is irradiated prior to detection of the amount of radiation impinging upon the film. As a result, the detector will detect amounts of radiation that can be significantly less than what has irradiated the film. Consequently, the reliability of an AEC is slightly compromised when used in a mammography machine.
Consequently, each mammography machine must have the AEC system frequently calibrated and adjusted on site by a service engineer. The process of calibration is tedious and expensive for many reasons. First, each cassette is slightly different, and thus, the same cassette must be used for multiple exposures. Secondly, radiographic films must be used from the same box to maximize the homogeneity of the film. Thirdly, significant variations can arise in the processor including chemical variation and temperature variation.
Thus, there is a need in the art for an apparatus and method that permit one sheet of radiographic film to be exposed multiple times including a simple and reliable method for determining the optical density of the film. However, such a solution must account for the decrease in the intensity of the x-ray field with an increase in distance from the source, known as the inverse square law. Optical density is also dependent upon the intensity of the x-ray field, such that there is additionally a need for an apparatus and method that compensates optical density measurements for changes due to the spatial variation.
The present invention provides an apparatus and method permitting one sheet of radiographic film to be exposed multiple times including a control unit for correcting the optical density of the film based upon the spatial variation of x-ray field intensity. In particular, the present invention includes a system for determining an optical density of radiographic imaging film comprising an emitter of soft x-ray radiation, a sheet of radiographic imaging film disposed a distance from the emitter, an attenuator simulating the radiological properties of a human breast, a mask for absorbing soft x-ray radiation disposed between the attenuator and the radiographic imaging film thereby creating a reference exposure, and a template for selectively irradiating the radiographic film.
The combination of the foregoing elements allows for multiple exposures on a single sheet of radiographic film. The mask defines a divide along its central axis that is coincident with the center portion of the film, thereby allowing the center portion of the film to be exposed during a reference exposure. The mask prohibits the exposure of two portions of the film that are laterally adjacent to the divide. The template is then aligned such that it permits selective irradiation of the portion of the film that was previously unexposed, and prohibits exposure of film that is yet to be exposed. The template is then moved such that previously unexposed film may be exposed, and again, the template prohibits exposure of film that is yet to be exposed. The foregoing process is repeated until there are enough sequential exposures to warrant processing the film. The template defines
The method of the present invention includes a step of irradiating a single sheet of radiographic film while simultaneously absorbing portions of radiation with a mask thereby preventing exposure of portions of the film. The mask defines a divide along its central portion, which allows the center of the film to be irradiated while preventing other portions from being irradiated. The unexposed portions of the film is selectively exposed multiple times utilizing a template having apertures for transmitting x-radiation such that film exposure occurs at various distances from the center of the film. The multiply and selectively exposed film is interpreted by a densitometer that measures the optical density of the film. The optical density data is corrected by the controller, which corrects the optical density measurements based upon the relationship between the distance between the point of interest and the center of the film.
The above and further objects of the present invention will more fully be apparent from the following detailed description with accompanying drawings.